The present invention generally relates to hardcopy apparatus, such as copiers, printers, scanners, facsimiles, and more particularly to improved media holddown devices for such apparatus.
To reduce the effects of paper curl and cockle on dot placement during printing, conventional practice is to employ sheet holddown devices such as electrostatic or suction devices. Cockle effect is the reluctance of the paper to bend smoothly. Instead it bends locally in a sharp fashion, creating permanent wrinkles.
In an electrostatic holddown device, for example, paper flatness is maintained by establishing electrostatic attraction between a flat support plate on the printer and the back surface of a sheet to be printed. Likewise, in vacuum holddown devices, sheet flatness is maintained by providing suction between a support plate and the back surface of a sheet to be printed. It should be noted that, in either type of holddown device, direct contact of the holddown device with the printed surface is avoided to minimise ink smearing and other adverse affects on print appearance.
Although conventional vacuum holddown devices are fairly effective in maintaining sheet flatness during printing, they have drawbacks. One drawback is the complexity of maintaining the same holddown force along the entire width of the medium while printing, i.e. in the direction of the printheads motion. This is due to the losses of air that the conventional devices allow, causing the medium to be subject to different forces, i.e. forcing the medium to rotate while it is advanced in the direction of the media motion.
This is particularly true in case of a large format printer, prior art printers, like ENCAD PROe, a 60xe2x80x3 wide printer, having a single vacuum channel/chamber extending along the entire platen may cause a number of drawbacks.
In fact, increasing the size of the platen implies the increase of the number of different media sizes which might be handled by the same printer. For instance, a printer having a platen 60xe2x80x3 wide may be used to handle not only media of the same nominal size, i.e. 60xe2x80x3, but also smaller width such as: 50xe2x80x3, 42xe2x80x3, 24xe2x80x3 or down to 8.5xe2x80x3.
So, when a medium having a smaller size in placed on the platen of such a printer two results may be obtained:
a) the vacuum holddown system is capable of apply enough negative pressure to the back of the medium to keep it flat on the platen, but due to the losses of air generated in the part of the platen not covered by the medium, the pressure is applied not uniformly in the printhead motion direction;
b) the dimension of the air losses caused by the smaller medium is so big that the air flow generated in the vacuum chamber not allow to apply a negative pressure to the back of the medium, sufficient to hold the medium flat on the platen.
A known method to overcome this problem has been disclosed in Falcon Color RJ-800C full color inkjet plotter available from Mutoh Industries Ltd.
This printer employs a single vacuum chamber connected to a fan having at the end, further from the fan, a wall which is slidable in a horizontal direction to increase or reduce the dimension of the vacuum chamber itself. A pinch roller is mounted on this wall, to engage a border of the medium. The opposite border of the media is engaged by a second fixed pinch roller, located at one end of the print zone. When a medium having a different size is loaded in the printer, the fixed pinch roller engage one side of the medium, while the other pinch roller is slid laterally in one or the other direction to engage the other side of the medium. By moving the pinch roller, the wall is slid too, so that the vacuum chamber result in being fully covered by the medium and thus reducing to the minimum the air losses that may cause the above drawbacks.
However, this arrangement requires manual engagement of the medium, which may result in damages to medium (ink transfer from hands to the medium or wrong engagement operation), loss of time, or wear of the wall which may cause substantial air losses.
The present invention seeks to provide an improved hardcopy apparatus and method of holding down a medium in the hardcopy apparatus.
According to an aspect of the present invention there is provided a hardcopy apparatus comprising a media drive roller and a media holddown unit, such holddown unit comprising a platen, and a vacuum source for generating negative pressure on at least a portion of a medium positioned on said platen to keep said portion substantially flat on said platen, wherein the holddown unit comprises at least two vacuum chambers, both in air communication with said vacuum source, each of said vacuum chambers being capable of applying a negative pressure to a different portion of the medium positioned on a corresponding different region of the platen.
This allows the hardcopy apparatus to handle a large variety of media sizes without affecting the capability of providing a fast, simple and clean loading system for the apparatus. Furthermore no manual movement of mechanical parts of the apparatus is required when a medium having different size from the previous one is loaded.
In addiction, the present invention can be particularly suitable to inkjet printers which preferably require a media to be periodically accurately indexed across a print zone defined in the printer for receiving the ink.
Preferably, the pressure generated in at least one of the two chambers is independent from the pressure generated in the remaining chamber or chambers. More preferably, the pressure generated in a number of the at least two chambers is substantially not affecting the uniformity of the pressure in another chamber of the at least two chambers.
In a preferred embodiment, a bypass conduit is employed to interface one of the at least two chambers to the vacuum source by passing the remaining chamber or chambers. Moreover, each but one of the at least two chambers is interfaced to the vacuum source by a substantially independent bypass conduit.
This helps to reduce any substantial air interference with the remaining vacuum chambers, so that a more uniform negative pressure can be applied to the back of the medium.
In a further preferred embodiment, at least one of the at least two chambers is connected to the correspondent bypass conduit via an aperture, substantially reducing the air circulation toward the vacuum source.
The reduction of air circulation in the chamber helps the vacuum source to generate higher depression in the vacuum chambers.
Preferably, the vacuum source comprises at least a fan and more preferably at least two fans in series, for maintaining a smaller diameter of the fan but increasing the total power of the vacuum source.
Typically, substantially the same pressure is applied to each different portion of the medium.
Viewing another aspect of the present invention there is provided a method for holding down a medium when placed on a platen in a hardcopy apparatus, comprising a vacuum source, comprising the steps of: placing the vacuum source into continuos and substantially independent air communication with a number of distinct regions of the platen, positioning the medium to cover one or more regions of the platen, applying a negative pressure to the back of the medium via the independent air communication between the vacuum source and each region of the platen covered by the medium.
Preferably, said negative pressure is substantially uniformly applied to the back of the medium.